US20070156060 discloses a real time video based automated mobile sleep monitoring system. A processor infers real-time information about the sleeping state of a subject from the video images, distinguishing between sleeping states. For adults, sleeping states like deep sleep, low rhythm breathing, abnormal movement, obstructive apnea and central apnea can be used as distinguishable sleeping states. US20070156060 discloses that the monitoring system can also be applied to a baby monitor. For infants states like sleeping, awake, standing up, lying on back, lying on stomach, moving, occasionally turning over, crying, thrashing and vomiting can be used as distinguishable states. The system may generate different sleep reports for adults or infants. But the detection operation is described in general without addressing problems that are particular to baby monitoring.
It is known to use image data correlation to determine the state of a subject. US20070156060 uses correlation between image content changes to determine sleeping state. A frame comparator compares pixel values between a pair of images obtained one second from each other. When the difference between image gradients at corresponding locations in images obtained at a pair of time points exceeds a threshold, motion is detected. This results in a map of image locations where motion has been detected for the pair of time points. The processor computes correlations between maps for different pairs of time points. The correlations are used to detect whether there is a temporally repetitive pattern. The detection results are used as an indication of sleep state.
Even though such techniques can provide some information, it remains a problem to increase the robustness and reliability of the determination of sleeping state. For babies in particular, detected motion is often not associated with turning: babies often make large movements with their arms and legs. Furthermore, babies move around during sleep far more than adults, who basically remain aligned with the length of the bed.